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Under the Hood: Troubleshooting 'multiple misfires' in motorhome

Brad Bergholdt, Tribune News Service on

Published in Business News

Q: I have a 2002 21-foot motorhome on a 3500 Chevrolet frame with a 5.7 engine. It now has 95,000 miles. At about 85,000 miles, the engine started losing power at driving speed. At first the engine would keep running but only at about 10 mph. After it sat overnight it would run fine. Now the engine will just lose power and quit. It will immediately start back up but will not run when put in gear.

It shows a computer code that says "multiple misfires." It has been tuned up and had its convertors cleaned, ECM replaced and the ignition coil replaced. This lasted 1,250 miles and now the problem is back. Could something be shorting out the coil? So far I have not found a mechanic that has seen this problem or knows how to fix it. Can you give me some help on finding out what is causing this?

-- Rick

A: This shouldn't be too tough to fix if the symptoms are present or can be induced. I'd start with a close visual inspection of the distributor ignition system's components and a scope check of the system's primary (coil control) and secondary (high voltage/spark) waveforms. A faulty pick-up sensor or ignition module would show up as an erratic primary waveform, and a check of secondary system required and available voltage can point out issues such as a faulty coil, cap, rotor, plug wires or plugs. I'm not sure what you meant by being "tuned up," if the mentioned secondary parts weren't replaced, they're possible offenders.

A great way to test an ignition system's ability to deliver is to employ a $20 spark tester such as the OTC 6589 or similar. The tool resembles a spark plug with a huge gap, and clips on to a grounded metal component. It can be temporarily installed on the end of a plug wire, coil wire or individual coil pack. If spark won't jump the tool when cranking or running, one works back toward the source to see why. Spark can either have a hard time getting to the end of a plug wire (excessive resistance/corrosion in cap/rotor/wire) or is weak or nonexistent due to spark leakage upstream due to an insulation fault or poor primary ignition performance.

It sounds as if the fault may appear when the engine is fully warmed up/hot, and/or perhaps there is just enough available secondary voltage (spark) to meet certain conditions but not all. In the first case, I'd try to induce the fault by running the engine with the hood closed, and/or drive it until the symptoms surface. I'd be suspicious of the pickup sensor and ignition module, as they both are subject to quite a bit of heat (attached to distributor metal) and can result in erratic primary system performance. Dirty/corroded pickup/module terminal connections have been a problem over two decades with this system. A flaky electrical connection or possible faulty ignition switch could also be limiting needed current to the coil and module; this is easily verified by testing for voltage drop. Insufficient but consistent spark may be a result of low primary current or faulty secondary parts (easily mitigated by replacement).

 

This is a problem that'll be fixed by somewhat old school yet highly skilled detective work, as opposed to a scan tool solution. It's unlikely your new coil would be damaged by anything other than a downstream high resistance fault. It may be of poor quality or wasn't the culprit in the first place. Were it not for confined access and likely working over a hot engine, this sounds like a delightful challenge!

About The Writer

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at bradbergholdt@gmail.com; he cannot make personal replies.

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